Stannous solutions containing hydroxy carboxylic acid ions their preparation and their use in plating tin on conductive surfaces particularly on aluminum

ABSTRACT

SOLUTIONS OF SIMPLE OR COMPLEX SALTS OF TIN AND GLUCONIC ACID OR SIMILAR POLYHYDROXY CARBOXYLIC ALIPHATIC ACID, MADE NEUTRAL OR NEARLY SO BY ADDITION OF A NONMETAL BASE SUCH AS AMMONIUM HYDROXIDE, ARE USEFUL IN FORMING A DENSE, ADHERENT TIN PLATE ON CONDUCTIVE SURFACES. THEY ARE PARTICULARLY USEFUL IN PLATING TIN METAL ON ALUMINUM OR ALUMINUM ALLOYS.

United States Patent Inventor I-Iarold P. Wilson Sewickley, Pa.

Filed Sept. 16, 1969 Patented Oct. 26, 1971 Assignee Vulcan Materials Company Clark, N.J.

Continuation-impart of application Ser. No. 790,157, Jan. 9, 1969, now abandoned.

STANNOUS SOLUTIONS CONTAINING l-IYDROXY CARBOXYLIC ACID IONS, THEIR PREPARATION, AND THEIR USE IN PLATING TIN 0N CONDUCTIVE SURFACES, PARTICULARLY ON ALUMINUM 19 Claims, No Drawings [56] References Cited UNITED STATES PATENTS 2,801,959 8/1957 Du Rose 204/45 3,082,157 3/1963 Francisco et al... 204/54 3,108,006 10/1963 Kenedi et a1 106/1 3,193,474 7/1965 Kenedi et a1 204/38 3,274,021 9/1966 Jongkind et a1 106/1 X FOREIGN PATENTS 208,507 10/1955 Australia 204/54 OTHER REFERENCES A. G. Gray, Modern Electroplating, pg. 422 (1953) Primary Examiner-G. L. Kaplan Attorney-Burns, Doane, Swecker & Mathis ABSTRACT: Solutions of simple or complex salts of tin and gluconic acid or similar polyhydroxy carboxylic aliphatic acid, made neutral or nearly so by addition ofa nonmetal base such as ammonium hydroxide, are useful in forming a dense, adherent tin plate on conductive surfaces. They are particularly useful in plating tin metal on aluminum or aluminum alloys.

STANNOUS SOLUTIONS CONTAINING I-IYDROXY CARBOXYLIC ACID IONS, THEIR PREPARATION, AND THEIR USE IN PLATIN G TIN ON CONDUCTIVE SURFACES, PARTICULARLY ON ALUMINUM CROSS-REFERENCE This application is a continuation-in-part of copending application Ser. No. 790,l57 filed Jan. 9, 1969, now abandoned.

BACKGROUND OF INVENTION There has been a long standing need for an effective process for electroplating tin on various conductive substrates, and particularly for plating it directly on aluminum and its alloys in particular. To provide a dense, smooth and strongly adherent coating, it has heretofore been necessary to preplate a reactive base metal such as aluminum with an intermediate layer of a metal such as copper before depositing a tin plate thereon. And even when tin has heretofore been electroplated on a relatively inactive substrate, the tin plate obtained from previously known electrolyte solutions has left considerable to be desired.

There are many reasons for wanting to plate a base metal such as aluminum with tin or a tin alloy. For example, aluminum bus bars are coated with tin to prevent interruption in the flow of electricity that a buildup of nonconductive aluminum oxide on their surface would bring about. Aluminum automotive bearings must be plated with a harder metal because aluminum itself galls badly under the mechanical stresses involved. However, processes heretofore available for this purpose have been either relatively complex and expensive or relatively ineffective in producing strongly adherent, high quality tin plate. The fact that different aluminum alloys behave differently toward the heretofore available tin electrolyte solutions has made tin plating a very difficult process to control and has made it virtually impossible to plate different alloys or combinations ofmetals in a single bath.

lmmersion plating of tin on base metals such as aluminum has been plagued by similar difficulties. For instance, as described in U.S. Pat. No. 2,947,639, when a sodium stannate solution is used to plate aluminum, sodium hydroxide forms in the process which tends to react with the aluminum substrate, liberating hydrogen gas in the process and thereby forming blisters. As suggested in this patent, this tendency to form blisters can be counteracted by including an alkali metal polyphosphate in the solution. However, even this expedient has not been found completely satisfactory as some occasional, localized blistering still is likely to occur and plate adhesion as well as plate density have not been as high as desired. More particularly, tin coatings produced in accordance with teachings of the prior art, when tested for their resistance to corrosion in percent NaCl solution at room temperature and to air saturated with water vapor at 100 C., showed that porosity of the tin plate coupled with residues of electrolyte, additives, impurities and reaction products on the aluminum surface contributed to the latent activity of the aluminum and led to the formation of blisters due to gas formed by electrolytic reactions with progressive destruction of the bond between the plate and the substrate. Even when the surface of the aluminum was kept as clean as possible and the original adhesion of the tin plate was adequate the porosity of the tin plate at high density still was sufficient to permit eventual breakdown of the tin coating under adverse conditions. This was true even when the tin plate was densit'red and polished to a mirror surface by intensive burnishing with tin metal or a highly polished metal such as nickel alloy.

In the art ofelectroplating other metals such as antimony or bismuth it has been heretofore suggested that certain improvements in plating quality could be obtained by including in the electrolyte solution various chelating agents such as tartaric acid or gluconic acid or phytic acid or alkali metal salts of ethylene diamine-tetraacetic acid, generally using an alkali metal hydroxide or an alkylolamine base to adjust the pH of the solution to at least 8.5 and preferably between 9 an ll.

U.S. Pats. Nos. 2,801,959 and 3,256,l60 are representative of these prior teachings. The deposition of tin on stainless steel from a solution comprising tin chloride, sodium phytate and sodium hydroxide at a pH of about 12 has also been disclosed in U.S. Pat. 2,973,308. However, attempts to use a similar electrolyte solution in plating tin on aluminum have failed to give good results, either because of an unwanted reaction between the alkali metal hydroxide and the aluminum substrate, or because of the inability of the chelating agent to hold the tin properly in solution, or because of a combination of these and other factors.

OBJECTS AND SUMMARY OF INVENTION It is an object of this invention to provide a stable tin plating solution capable of producing smooth, continuous, dense and strongly adherent tin deposits on conductive surfaces. A more particular object is to provide such a solution which is capable of producing a continuous, corrosion resistant, strongly adherent tin coating on aluminum and aluminum alloys.

A further object is to provide an improved process for plating tin on conductive surfaces and especially on aluminum or aluminum alloys.

The foregoing and other objects, it has been discovered, can be attained by formulating tin plating solutions from tin salts such as stannous chloride or stannous sulfate by including ions of gluconic acid or a similar chelating polyhydroxy aliphatic acid or an ester thereof in such a solution and adjusting its pH approximately to neutrality by addition of a nonmetal base, especially ammonium hydroxide or an alkylolamine base such as an alkanolamine or an alkanolammonium hydroxide.

In the case of electroplating, particularly good results are obtained by first forming a very thin tin coat by immersion prior to turning on the electric current.

DESCRIPTION OF INVENTION It has now been found that a clear, stable, fairly concentrated electrolyte solution can be prepared which contains either a simple or a complex stannous salt ofa chelating sugar acid, e.g., gluconic acid. Such a solution should contain from about 10 to 70 grams per liter, preferably from 30 to 60 g.p.l., of dissolved stannous tin and from 1.8 to 3 or 4 or more mole equivalents of the chelating sugar acid per mole of tin, and have its pH adjusted to a value within the range between about 6 and 8 by addition of a nonmetal base such as ammonium hydroxide or an alkylolamine base of 2 to 8 carbon atoms per molecule. Mono-, diand triethanolamine, monoand dipropanolamine and tetraethanolammonium hydroxide are examples of suitable alkylolamine bases. For instance, at a mole ratio of 1.8:] gluconic acid to tin it is possible to make a stable aqueous solution containing 50 percent or more stannous gluconate, and the stability of such a solution for electrotinplating may be further improved by adding at least 10 percent excess of the stoichiometric amount of gluconic acid. When evaporated under vacuum, such a solution may be further concentrated and a hydrated crystalline or dry solid form of stannous gluconate may eventually be produced.

While gluconic acid, C H,,(OH) COOH, is a particularly effective source of the required chelating anions, similarly functioning chelating hydroxy carboxylic acid derivatives of glucose or of similar monosaccharides, e.g., alpha-glucoheptonic acid, or the lower esters or lactones of such chelating sugar acids, may also be used, individually or in mixtures. Such chelating compounds desirably contain from 3 to 7, or preferably 5 to 7, carbon atoms, 1 to 2 carboxyl groups and 2 to 6, or preferably 4 to 6, hydroxy! groups per molecule.

Suitable examples include: glyceric acid, CH,OHCHOH- COOH; the pentonic acids such as dor l-arabonic, xylonic, ribonic and lyxonic acids; the hexonic acids such as gluconic, mannonic, galactonic, talonic, altronic, allonic and idonic acids; and the heptonic acids, notably alpha-d-glucoheptonic acid and beta-d-glucoheptonic acid lactone; i.e., monocarboxylic polyhydroxy acids; as well as dicarboxylic hydroxy acids such as glucaric, saccharic and tartaric acids. All are referred to herein under the generic term chelating sugar acid, and the monocarboxylic polyhydroxy carboxylic aliphatic acids are referred to under the term "aldonic acid. Next to gluconic acid, glucoheptonic acid is most preferred.

Generally these solutions of stannous gluconate or stannous salt of chelating hydroxy carboxylic acid are most effective at tin concentrations of about 50 g.p.l. However, it is possible to lower the tin concentration to around 20 g.p.l. or even down to g.p.l. Higher concentrations up to about 70 or even 100 g.p.l. may also be used, but at high concentrations supersaturation may occur and a complex tin salt may eventually crystallize out. Solutions containing tin concentrations substantially below 50 g.p.l., e.g., 25 g.p.l., require a more careful determination of optimum cathode current density in order to obtain high tin plate quality in any given case.

The solutions are somewhat sensitive to temperature and work best in the range of from to 30 C. for electrotinplates of good quality. The mole ratio of sugar acid to tin affects somewhat this temperature sensitivity. As the mole ratio of sugar acid is increased, the solution temperature which is safe for the production of good tin plate quality at normal cathode current densities may be raised. For instance, at a ratio of 6 moles of gluconic acid to 1 mole of tin the solution temperature may be raised to 70 C. higher.

The preparation of stannous hydroxy carboxylate initially was found to be very difficult. However, a satisfactory solution was prepared by dissolving stannous chloride at a tin concentration on the order of about 25 to 50 g.p.l. in a water solution containing about 1.8 to 3.0, preferably 2.2 to 2.6, moles of gluconic acid or other chelating sugar acid per mole of stannous chloride added, and then adding concentrated ammonium hydroxide or alkylolamine base solution to the agitated solution until a pH between about 6 and 8, preferably between 6.5 and 7.5 and most preferably between 6.2 and 6.8, was reached and maintained. During the addition of the base the solution temperature may rise to about 50 C. or more without any ill effect.

During electrolysis occasional adjustments of the pH of the solution may be made to maintain it at the optimum level by addition of nonmetal base or sugar acid, depending on whether the pH of the solution tends to drift toward the acid or the alkaline side. Additions of strong inorganic acids such as hydrochloric or sulfuric, or of strong bases containing an alkali metal, are not advisable as the activity of the aluminum cathode in such a case tends to increase excessively as the concentration of such strong anions increases. Weak acids such as acetic may be added to maintain the proper pH, though the addition of sugar acid is preferable. An exception found was boric acid, H B0;, which may be added in concentrations up to 10 g.p.l. or even higher and actually tends to benefit the fineness and adhesion of the resulting tin plate.

Complex electrolyte solutions of a generally similar character can also be made with other water soluble salts of tin such as stannous sulfate, stannous fluoborate or stannous fluoride. Best results are most readily obtained when stannous chloride or stannous sulfate is used as the source of tin and such solutions are therefore particularly advantageous, while solutions formulated from stannous fluoride are the least desirable.

The complex electrolyte formulated in accordance with this invention from stannous chloride, gluconic acid and ammonium hydroxide at a solution pH of 7.0 (measured at 25 C.) may be represented by the formula {Sn(gluconic acid),(NH BJCL When stannous sulfate is used to make the complex solution, the fundamental composition of such a complex cgrnpouruiat a pH of 7 can be represented by the formula the chelating sugar acid may be added in such a case in view of the added presence of nickel or other metal ions used to form the desired tin alloy.

The electrolyte of the present invention can be used to form good quality electrotinplates on other conductive surfaces besides aluminum. For instance, the substrate may be a metal casting and assembly or other metal articles fabricated from aluminum, aluminum alloys, iron, nickel, stainless steel, zinc, copper, etc., or from a combination of two or more of any such metals. An exception is magnesium and alloys having a very high magnesium content. Magnesium is too active and displaces tin with evolution of hydrogen from the complex electrolyte solutions even if the pH is as high as 9.0. The arti cles being plated may be in various forms, simple or complex shapes, flat sheets of limited area, discs, wires and so on. The tin plates produced are bright, dense, soft, ductile, and generally have good adhesion to the base metal.

To obtain a good quality electrotinplate and to maintain good adhesion of the plate without blistering, it is essential to attain a chemically clean substrate surface with no oxide, hydroxide or any impurity present just before plating. Satisfactory cleaning of aluminum may be obtained, for instance, by immersion of the aluminum in a hot solution of an alkali detergent compound, such as a silicate-free sodium alkylbenzene sulfonate, followed by etching at ambient temperature in a caustic sodium hydroxide solution (e.g. 10 percent concentration), and finally etching in a nonoxidizing acid such as dilute hydrochloric acid solution, preferably one containing 1 to 2 percent of a chelating agent such as citric or gluconic acid. Conventional nonionic wetting agents, such as ethoxylated alkyl phenols, may be included in such cleaning solutions. Washing with distilled water after each step is helpful. The use of oxidizing alkali detergents or etching acids having any degree of oxidizing power, such as nitric acid, should be avoided to preclude fonnation of any oxidation residue on the aluminum surface that tends to promote local action and consequently blistering of the tin plate.

Following the electrotinplating, the plated article is desirably washed with distilled or otherwise demineralized water. Boiling distilled water may be used. Drying the plated article in a hot air oven at C. is useful in bringing about complete dehydration and at the same time testing the stability of the tin plate. However, when the tin plate is to be hot flowed, only limited air drying, e.g., at 50 to 60 C., suffices. The formation of aluminum oxide at high temperature may spoil the adhesion of the tin plate.

The tin coatings tend to be slightly crystalline and consequently produce surfaces that are not normally highly brilliant. This type of tin plate is particularly useful where resistance to corrosion and abrasion, or an improvement of surface lubrication properties are desired. The tin plate may be polished or burnished so as to reduce residual porosity to a minimum. Burnishing the tin plate with soft tin metal can be sued to produce a mirror finish. Flow brightening of the tin plate on aluminum may be achieved if the surface temperature is maintained close to the melting point of the tin and the time of exposure to heat is kept at a minimum. Excessively long exposure of the tin plate to high temperature may cause the formation of a brittle grey tin-aluminum alloy and the breaking away of excess tin in the form of beads. In contrast to the successful flow brightening of tin plate formulated in accordance with the present invention, such flow brightening is impossible when the tin plate is formed from an electrolyte solution containing any amount of an alkali metal such as sodium or potassium, as surface residues of such an alkali metal between the aluminum and the tin cause the melted tin plate literally to explode away from the aluminum.

To obtain tin plate of good quality, it is desirable to keep the cathode current density in the range of from 10 to 30 amperes per square foot, preferably between 15 and 25 amperes per square foot. At higher current densities there is a tendency for the tin plate to become excessively crystalline or spongy unless special measures are taken to prevent this, such as causing the cathode surface to move rapidly relative to the electrolyte solution. However, in the case of aluminum wires (e.g., 16-18 gauge) or other shapes having a comparably high surface/volume ratio a cathode current density of 40 amperes/square foot and higher is permissible. Particularly when such wires move through the electrolyte solution at high speed current densities of 100 amperes/square foot or even more can be used with satisfactory results.

A suitable current density is best determined for each specific situation by preliminary routine screening tests. Cathode current efficiencies in the order of 90 percent and up may be readily achieved. When the electrolyte solution is agitated while the plating operation is in progress, it is advisable to conduct the agitation in a manner such that the air bubbles are not substantially entrained in the solution. Air in contact with the cathode tends to spoil the quality of the tin plate, particularly its homogeneity and adhesion.

For best results, the ratio of the area of the tin anode to the area of the cathode being plated should be at least 2:1. A ratio of about 3:1 is preferred in typical operations, but higher ratios may be used.

The fineness and brightness of the tin plate from the complex electrolyte solutions of this invention can be further improved in the usual manner by the addition of various brightening and conditioning agents. It is important, however, not to add such agents in the form of their alkali metal salts. Generally such brightening and conditioning agents may be added in a concentration between about 0.5 and grams per liter. Orthocresotinic acid or beta-naphthol at between about 0.5 (e.g., 2.0 g.p.I. are very effective. Water soluble polyalkylene glycols such as the soluble polyethylene glycols (e.g., Carbowax 1500," M.W. about 1,500) or polypropylene glycols at a concentration of from between 0.5 to 1.0 g.p.l. or higher are suitable. These polyglycols typically have a molecular weight from about 900 to about 6000 butpolyglycols of higher or lower molecular weight may be used. Dimethyl formamide is an effective brightening and conditioning agent in concentrations of from 2 to 10 g.p.l. Small concentrations of alkyl or aryl amines, e.g., triethylamine, diphenyl amine and the like, also can be used to enhance tin plate quality. Conventional additives that tend to coat the cathode, e.g., wood tar extracts, aldehyde-amine complexes or any other substance that tends to form tar or a semisoluble polymeric suspensoid, should be avoided. Such coating of the cathode is undesirable in that it promotes local galvanic action with consequent gas formation, blistering and loss of adhesion of the tin plate.

Described below in table l is the composition of complex electrolyte solutions formulated from stannous chloride in accordance with this invention which give good performance in electroplating tin on aluminum or its alloys. Unless otherwise indicated, all proportions and percentages are expressed on a weight basis through this specification.

TABLE I Complex Electrolyte from Stannous Chloride and Gluconic Acid Similarly, effective electrolyte solutions can be formulated from stannous sulfate according to the formulation shown in table 11.

TABLE II COMPLEX ELECTROLYTE FROM STANNOUS SULFATE AND GLUCONIC ACID Concentration Examples of still other formulations of complex gluconic acid electrolyte solutions are summarized in tables 111 and IV below, using stannous fluoborate and stannous fluoride, respectively, as the initial soluble tin salt. In each case the specified amount of tin salt solution was added to the 50 percent gluconic acid solution and then the ammonia added to give the desired pH. The remaining additives were put into solution thereafter.

TABLE III COMPLEX ELECTROLYTE FROM TIN FLUOBORATE AND GLUCONIC ACID Component Concentration Aqueous Sn(BF,), solution 250 g.p.l. (49% concentration) Aqueous gluconic acid solution 331 g.p.l. (50% concentration) Aqueous ammonia 1S2 ml./|. (29% NH,) Polyethylene glycol 1.0 g.p.l. .(Carbowax 1500) Beta-naphtha] 1.0 g.p.l. Hydroquinone 1.0 g.p.l.

The solution described in table 111 contained 50 grams tin (Sn) per liter and 2 moles gluconic acid per mole of tin, and had a pH of 6.8.

With consumable high grade tin anodes, a fair electrotinplate was formed on aluminum from this solution at room temperature (25 C.) at a current density of 18.1 amperes/square foot and at a cell potential of 0.48 volts. However, the plate quality was not as good as in the case of the solutions formed from stannous chloride or stannous sulfate. With the stannous fluoborate solution there was some evidence of latent activity with the formation of some tiny blisters. In the absence of any imposed electric current, clean aluminum showed observable activity in this solution with evidence of some immersion tin plating.

TABLE IV COMPLEX ELECTROLYTE FROM STANNOUS FLUORIDE AND GLUCONlC ACID Composition Concentration 50F, 66 g.p.l. Aqueous gluconic acid solution 33l g.p.l. (50% concentration) Aqueous ammonia 88 mlJl. (29% NH.) Polyethylene glycol 1.0 g.p.l. (Carbowax I500) Beta-naphthol l.0 g.p.l. Hydroquinone 1.0 g.p.l.

This solution contained 50 grams tin per liter and 2 moles gluconic acid per mole of tin, and had a pH bf 6.7. it was somewhat more stable than the fluoborate solution described in table Ill but the safe pH range did not extend substantially beyond 7.5 and there was a tendency to precipitation at pH 8.0. Electrotinplates as well as immersion tin plates formed on aluminum from this solution were of distinctly poorer quality than those from the fluoborate solution and very much poorer than those from either the stannous chloride or the stannous sulfate solutions.

When the sulfate salt is used as illustrated in table ll, it is advisable to exclude strong alkali metals from the electrolyte solution the same as when stannous chloride is used, but in addition it is advisable to exclude as much as possible all other inorganic anions such as any halide, nitrate, etc. The borate ion, however, may be beneficial in improving the fineness and adhesion of the tin plate. For this purpose, therefore, boric acid, H 80 may be added in concentrations from about 0.5 to g.p.l. or even higher. Other compatible conditioning agents of the type described earlier herein and otherwise well known in the art may also be included in the stannous sulfate type as well as in the other complex stannous salt solutions similarly as in the first described stannous chloride type solutions.

lnstead of making a complex stannous salt solution as described above, it is also within the scope of this invention to make chelated stannous salt electrolyte solutions using essentially pure stannous gluconate as the starting material. in such a case it may be advantageous to improve the conductivity of such a solution by adding to it 1 to 40 grams per liter, preferably 10 to g.p.l., of an ammonium salt of high ionic strength such as ammonium sulfate or ammonium chloride. Such an addition is beneficial regardless of whether or not excess sugar acid is present. When it is desired to increase the conductivity of the solution so as to lower the cell potential, it is possible to add more conductivity salt, e.g., as much as about one-half gram mole of conductivity salt per liter of solution. In the case of ammonium chloride even as much as 1 gram mole per liter may be added. At such high salt concentrations the solutions formulated starting with pure stannous gluconate are approximately equivalent to the complex electrolyte solutions described earlier herein.

On the other hand, preparation of the electrolyte solution starting with substantially pure stannous gluconate or an equivalent salt of another sugar acid has the advantage of permitting limitation of the concentration of any conductivity salt to any level that may be desired from the viewpoint of minimizing the activity of the aluminum or similar substrate. By thus permitting the formulation of an electrolyte solution having a composition different from that of the salts that would be formed inherently when a complex stannous gluconate solution is formulated directly starting with stannous chloride or sulfate, the use of stannous gluconate per se gives the electroplater additional freedom in controlling the conductivity of his plating bath.

Incidentally, when making the stannous gluconate or similar sugar acid salt by the addition of ammonia to a solution of a soluble inorganic stannous salt and subsequent conversion of the resulting stannous hydroxide precipitate to the desired stannous gluconate or similar chelate, there is a tendency for the amorphous white hydroxide to convert to the undesirable blue-black crystalline oxide. This is particularly likely when the solution pH is substantially on the acid side, e.g., in the range of from 4 to 6. The blue-black oxide does not react with the sugar acids appreciably at moderate temperatures, though it does react with such acids and dissolves to form a clear solution when the liquid mixture is maintained at a temperature above 75 C., e.g., C., for an extended time.

The formation of the blue-black oxide may be inhibited, it has been found, by adding ethylene glycol or propylene glycol to the stannous salt solution before addition of the ammonia. For instance, the addition of about 2 to 5 moles, preferably 2.l to 2.5 moles, of such alkylene glycol per mole of stannous salt in solution has been found surprisingly efiective.

Still more preferably, however, the risk of conversion to the black oxide can be limited and the need for an expensive conversion inhibitor such as ethylene glycol can be eliminated by adding an aqueous solution of stannous chloride to an agitated ammonia solution containing a sufficient excess of ammonium hydroxide to precipitate the stannous hydroxide in an alkaline solution to give a pH near 9, e.g., 8.5-l 0.

The reaction between the ammonia and the stannous salt heats the resulting slurry to approximately 50 C. and the agitated slurry is thereafter preferably heated further to 60 C. in as short a time as possible to grain the precipitate. Thereupen the solution is cooled. The white precipitate is filtered and washed thoroughly with distilled water and then dissolved at room temperature in sufficient 50 percentsugar acid, e.g., gluconic acid, for a ratio of between 1.8 and about 3.0 moles of the chelating acid per mole of tin. A clear yellow solution of the stannous chelate salt is thus produced. If any white or grey residue is present it may be dissolved by agitating and heating the solution.

When it is desired for the chelated stannous salt solution to contain a certain concentration of ammonium chloride the procedure described above may be modified eliminating the filtration step. lnstead, after the stannous hydroxide is precipitated it is allowed to settle and sufficient clear supernatant solution is decanted so that the remaining solution contains ammonium chloride equivalent to the desired concentration in the final electrolyte solution. An appropriate amount of the chelating acid in such a case simply is added to the slurry of precipitate in ammonium chloride solution and the stannous hydroxide is thereby completely dissolved.

ln accordance with the present invention electrolyte solutions are prepared from such chelated tin solutions by adding sufficient ammonium hydroxide thereto so as to raise their pH and make them neutral, or nearly so, by diluting them to the desired tin concentration, and by adding thereto a conductivity salt and one or more of the usual brightening and conditioning or leveling agents, etc., all as described earlier herein. For example, l to 2 g.p.l. polyethylene glycol (Carbowax i500), 0 to 2 g.p.l. beta-naphthol, 0 to 2 g.p.l. o-cresotinic acid. C to 2 g.p.l. 2-naphthol-6-sulfonic acid and 0 to 5 mL/l. of dimethyl formamide may be added to enhance the properties of the stannous electrolyte solution and of the final tin plate. A typical satisfactory solution of this kind is shown more particularly in table V below.

In electroplating tin or aluminum from electrolyte solutions of the kind described herein a cell potential ranging from about 1.1 to about 1.5 volts has been found suitable. However, the optimum value may vary considerably from case to case depending on the electrochemical characteristics such as specific resistance of the particular solution, the operating temperature and the physical arrangement of the particular apparatus used. Consequently, an actual working potential above or below the range cited may give satisfactory results when working under different conditions. The optimum value is readily established for each specific case by routine preliminary tests. At any rate, under proper conditions tin plate of good matte quality and good adhesion may thus be electroplated on a substrate such as aluminum in thicknesses which typically may range from about 0.4 to 1.0 mil.

available under the brand name Seqlene 540 from the A. E. Staley Company, for instance.

In contrast to gluconic acid or equivalent sugar acids mentioned above, other chelating acids have failed to give satisfactory results. Among the chelating acids which have been found unsatisfactory are citric acid, glycolic acid, malonic acid, succinic acid as well as the ammonium salt of ethylene diamine tetraacetic acid, mainly because of poor stability and the formation of precipitates. An attempt to use succinic acid was unsuccessful because of its insufficient solubility.

Brightening and conditioning agents which may be used in all of the types of the new electrolyte solutions described above include the following:

Run No.

Solution No I I V V Solution pH 6.9 6.9 7.0 7.0 6.7.

Cathode Sheet aluminum Length of inches of Sheet aluminum Sheet aluminum Length 019.5 inches rectangle. 16-18 gage rectangle. rectangle. 16-18 gage aluminum wire. aluminum wire.

Cathode area, cm. 9.0 25 25 8.7.

Solution temperature, C 28. 0.. 25.0 27 0 26. 5.

Time, min 25... 12 12.

Cell potential, volts 1. 03 0. 0.80.

Cathode current density, amp/123.. 18.1 40.8.. 18 1 21.4.

Nominal tin plate thickness, mils 0.95 1.01.... 1.0 1.12 0 53.

Tin plate quality- Bright and very Bright white wlth Smooth white with Smooth white with ery fine, smooth fine with good good adhesion. light grey tone improved and white. also adhesion. and good adhesion. ductile and adhesion. adherent.

4 mL/l. of dimethyl formamide was added to this solution.

' Equivalent lo 50 g. Sn/l.

" Snlgluconic acid mole ratio= 0S.

ILLUSTRATIVE PLATING OPERATIONS To further illustrate the invention, table V1 is presented below which summarizes the results of plating runs wherein the electrolyte described above in table 1 (Solution No. I") and the electrolyte described above in table V (Solution No. V) were used to tin plate samples of sheet aluminum and of aluminum wire under the representative conditions specified.

The tabulated data are indicative of very good quality of electrotinplate formed directly on an aluminum substrate. Run No. 2 shows the feasibility of operating at very high current densities and high rate of tin deposition when plating wire. In all cases good smoothness and adhesion of the plate were obtained, but particularly good adhesion was obtained when a small amount of dimethyl formamide was included in the plating bathv Similarly excellent results are obtained when the gluconic acid is replaced with an equivalent molar amount of alpha-glucoheptonic acid, CH,OH(Cl-1OH) -COOH. It can be obtained by acidicification of its sodium salt which is commercially l-lydroquinone, to improve brightness, fineness and adhesion.

Ethylene glycol, to improve solubility oflow solubility additives such as beta-naphthol, and enhance brightness, fineness and adhesion.

2-naphthol-6-sulfonic acid or its ammonium salt, a wetting agent which increases covering power, brightness, fineness and adhesion.

Formamide, greatly improves the brightness, fineness and especially adhesion of the tin plate with outstanding improvement in the quality and adhesion of flame flowed tin plates.

Dimethyl formamide, similar to formamide but even more effective.

Polyalkylene glycols, improve fineness and brightness.

As indicated above, formamide and especially dimethyl formamide have displayed remarkable effects on the electrolyte solutions and the electrotinplates on aluminum. Their solvent and electrolyte properties increased the solubility of other additives such as beta-naphthol and their effects on tin plate quality and greatly improved the adhesion of the tin plate to aluminum, particularly for the flame flowed tin plate. Other homologs of the formamide series may be used similarly.

Among the wetting agents, 2-naphthol-6-sulfonic acid and its ammonium salt are particularly preferred because of their high effectiveness in increasing covering power, brightness, fineness and adhesion. However, other wetting agents known in the metal plating art may be used similarly, e.g., cresol sulfonic acid, beta-naphthol sulfonic acid, the ammonium salts of same, etc., etc. The only kind of wetting agent which has been found to be undesirable for use in connection with the present invention are alkali metal containing compounds.

Beta-naphthol and orthocresotinic acid together have a synergistic effect in benefitting the fineness and brightness of the electrotinplate on aluminum, Small amounts, e.g., 0.005 to 0.5 g.p.l., of gelatin or glue also can be very beneficial.

When a tin plate of particularly high quality in terms of adhesion and absence of porosity is desired, various auxiliary expedients can be resorted to. For instance, greatly improved adhesion of the electrotinplate can be obtained by first immersing the clean etched aluminum in the tin electrolyte solution for at least 1 minute, e.g., for l to 10 minutes, before turning on the plating current. An almost imperceptible layer of tin deposited in the galvanic preplating step by a nongassing immersion plating reaction is surprisingly effective in improving the quality of the electrotinplate deposited thereon.

Preplating with copper is beneficial when maximum adhesion is wanted. For instance, this improves the ease with which the tin plate can be flow brightened and increases corrosion resistance of the brightened tin. Preplating with copper also permits the resulting product to be readily and effectively soldered.

Tin plate quality can also be improved by rubbing a tin cathode at its point contact over the thoroughly air dried electrotinplate while conducting direct current of l to amperes, -e.g., 8 amperes, to the tin plate as an anode and thus releasing a considerable amount of heat. This semiwelding treatment improves the bond of the tin plate to the aluminum. The Edirection of the direct current can be reversed making the tin plate the cathode, but the bonding is usually less effective with ethlS arrangement. As still another alternative, alternating current can be used in such semiwelding treatment.

A very significant reduction of tin plate porosity can also be .obtained by applying a reducing natural gas flame to the tin plate after it has been air dried or by using infrared or especially induction, microwave or radio frequency heating, or any other existing electronic heating method which causes the tin plate to flow and become strongly bonded to the aluminum substrate. Heating the tin plate to a temperature above the melting point of the plated tin or tin alloy, e.g., to a temperature between about 232 and 280 C. is satisfactory. To obtain a substantially even, homogeneous tin coating it is desirable to control the melting time as short as possible. The optimum treating is readily determined for each type of tin plate by routine preliminary tests.

Heating is an atmosphere of nitrogen or other inert gas such as argon is beneficial, or the heating may be conducted under substantially reduced pressure so as to minimize any deleterious effect of oxygen.

Two or more of the treating methods described above may be combined for producing the desired final improvement in tin plate quality. In fact, the semiweld treatment described above may be modified so that the welding current simultaneously accomplishes the desired flowing of the tin plate.

Electric resistance heating may also be used to fuse and flow 'the deposited tin plate on the conductive substrate. Plasma flame torches, ultrasonic welding and explosive bonding may also be used.

The corrosion resistance of the electrotinplate may also be very greatly increased without appreciable loss of electric conductivity by applying a solution of any convenient insulating resin or polymer to the tin coating and then baking the tin plate. For instance, electrotinplates formed on aluminum in a thickness of between about 0.4 and 1.0 mils, after being air dried, may be clipped once or several times, with intermediate air drying, in solutions or dispersion containing 110 to 40 grams/liter of acrylic or polyvinyl-acrylic resin (such as polym ethyl-methacrylate or a vinylidene chlorideacrylonitrile copolymer), in a suitable solvent such as methyl ethyl ketone or methyl isobutyl ketone. After such dipping the tin plate may be baked in a hot air oven at a temperature in the range from about 100 to 150 C. As long as the area density of the resin is minimized the electric conductivity on the tin plate remains substantially unimpaired.

In comparison with untreated electrotinplates on aluminum, the resin coated tin plates show exceptionally high resistance to corrosion even when exposed to an aerated agitated NaCl solution and the specimen being tested is coupled by wire to a copper anode to increase the severity of the test.

Other resins suiia polyethylene in ketone solution as well as other insulating resins such as phenol-formaldehyde condensates, polyurethanes, epoxy resins, polyvinyl chloride, polystyrene, silicone resins and so forth may be used similarly. However, chlorinated hydrocarbon solvents such as methylene chloride or trichloroethylene should be avoided as solvents for the resin being applied, as such chlorinated solvents are too active 9 P rases??? sews ssrat tiet i e tin plate with loss of resistance to corrosion. Use of oxygencontaining solvents such as the lower ketones, alcohols such as methanol, isopropyl alcohol or amyl alcohol, as well as the glycol ethers is preferred. Of course, an appropriate solvent must be selected for each resin so as to make certain that the resin used will readily dissolve therein.

Although the electrolyte solutions disclosed herein have been developed principally for electrolytic or galvanic plating of tin directly on aluminum, they are also useful for plating in multiple applications where intermediate layers of metals such as copper are plated on the aluminum to lower the activity of the base metal. The low activity of the nearly neutral solutions of the present invention may be valuable for limiting latent activity in tin plate on intermediate plates, thus promoting a more stable tin coating. These properties of the electrolyte solutions are also beneficial in plating tin on conditioned plastics where very corrosive or active solutions are not desirable.

For instance, the novel electrolytes disclosed herein may be used to plate plastics of the acrylonitrile-butadiene-styrene copolymer type (ABS" resins) after such plastics have been conditioned in an otherwise well-known manner, e.g., by treatment with stannous chloride solution. Preferably such conditioned plastics are preplated by electroless copper plating before the tin coating is applied by electroplating.

It should be understood that the foregoing description and examples have been presented principally for purposes of illustration and not for purposes of iimitation. The invention for which protection is desired is particularly pointed out in the appended claims.

lclaim:

I. An aqueous solution for plating tin on a conductive substrate, which solution is essentially free from alkali metal ions and comprises (a) stannous ions in an amount equal to between about 10 to 70 grams tin per liter, (b) as an essentially sole chelating agent, a chelating sugar acid in an amount of at least 1.8 mole equivalents per mole of said tin, and (c) a nonmetal base in an amount sufficient to give a pH between about 6 and 8.

2. An aqueous solution according to claim 1 wherein said sugar acid is gluconic acid and said base is ammonia, and wherein said solution further comprises anions of a strong inorganic acid selected from the class consisting of hydrochloric and sulfuric acid in a concentration equivalent to between about 1 and 40 grams per liter of the corresponding ammonium salt.

3. An aqueous solution according to claim 2 wherein said inorganic acid anions are chloride ions and wherein said solution further comprises from about 0.5 to 10 grams per liter of a compound selected from the group consisting of hydroquinone, o-cresotinic acid, beta-naththol, ethylene gylcol, propylene glycol, water-soluble polyalkylene glycols and formamides.

4. An aqueous solution according to claim 2 which further comprises from 2 to 10 grams per liter ofdimethyl formamide.

5. An aqueous solution according to claim 2 which further comprises from about 0.5 to 10 grams per liter of boric acid.

6. An aqueous solution according to claim 1 wherein said sugar acid is a monocarboxylic polyhydroxy aliphatic acid of 5 to 7 carbon atoms and 4 to 6 hydroxyl groups, and is present in an amount between about 2.2 and about 3 moles per mole of tin in the solution; wherein said base is selected from the group consisting of ammonia and alkylolamines of2 to 8 carbon atoms and is present in an amount sufficient to give a pH between about 6.5 and 7.5.

7. A process for preparing a tin electrolyte solution which comprises dissolving in water (a) about 50 to grams per liter of a tin salt selected from the group consisting of stannous chloride, stannous sulfate, stannous fluoborate and stannous fluoride, (b) 1.8 to 3 mole equivalents of a chelating acid selected from the group consisting of glyceric acid and C to C hydroxy carboxylic aliphatic acids containing 4 to 6 hydroxyl groups and l to 2 carboxyl groups per molecule per mole of stannous salt, and (c) sufficient base selected from the group consisting of ammonia and alkylolamines of 2 to 8 carbon atoms to adjust the pH of the solution to between about 6 and 8.

8. A process according to claim 7 wherein said tin salt is stannous chloride, said sugar acid is gluconic acid and said base is ammonia.

9. A process for preparing a tin electrolyte solution which comprises dissolving stannous chloride or sulfate in water, mixing the resulting stannous salt solution with ammonium hydroxide to precipitate stannous hydroxide from solution, mechanically separating the precipitated stannous hydroxide from the solution, redissolving the separated stannous hydroxide in an aqueous solution containing about 2.0 to 2.6 moles gluconic acid per mole of stannous hydroxide, and adding ammonium hydroxide to the resulting stannous gluconate solution in an amount sufficient to produce a substantially neutral solution characterized by a pH between about 6 and 8.

10. A process according to claim 9 wherein the substantially neutral stannous gluconate solution is concentrated by evaporation.

11. A process according to claim 9 wherein light colored stannous hydroxide is formed by adding the stannous salt solution to an agitated aqueous ammonia solution which contains sufficient ammonium hydroxide to convert the stannous salt to a precipitate of stannous hydroxide and give a pH between about 8.5 and 10 in the supernatant liquid.

12. A process according to claim 9 wherein about 2 to moles ethylene glycol is added to the stannous salt solution prior to mixing it with ammonia.

13. A process for plating tin on an aluminum or aluminum alloy substrate which comprises immersing a tin anode and said substrate as a cathode in a plating solution which is essentially free from alkali metals and comprises stannous ions in a concentration of from about 10 to 70 grams tin per liter and, as an essentially sole chelating agent, a chelating sugar acid in a concentration equivalent of at least 1.8 mole equivalents of acid per mole of tin and a sufficient amount of ammonium hydroxide to give a pH between about 6 and 8.

14. A process according to claim 13 wherein said sugar acid is a C to C monocarboxylic polyhydroxy aliphatic acid and electric current is set up between it and said anode to give a cathode current density of between about 10 and amperes per square foot.

15. A process according to claim 14 wherein said plating solution comprises stannous ions in a concentration of from about 30 to 60 grams tin per liter and ions of gluconic acid in a concentration equivalent to between about 2.2 and4 mole equivalents of acid per mole of tin.

16. A process according to claim 15 wherein an initial thin tin layer is deposited on said aluminum substrate from said plating solution by immersion and galvanic current action and further tin is deposited on said initial layer electrolytically in a system wherein said immersed aluminum substrate is connected as an anode and tin metal is connected as a cathode and current is passed through the system at a cathode current density of between about 10 and 30 amperes per square foot.

17. A process according to claim 15 wherein the tin anode and the aluminum cathode have a ratio of surface areas of between about 2:1 and 3:1.

18. A process according to claim 15 wherein the plating solution further comprises small effective amounts of ammonium chloride as a conductivity salt and a formamide as a brightener.

19. A process according to claim 15 wherein the substrate is an aluminum wire and the current density is between about 40 and 100 amperes per square foot.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N .0 Dated October 26,

Inventor(s) Harold Wilson It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On the page containing the Abstract, the line immediately following the inventor's address insert [21] Appl. No. 858, 544

Column 12, line 35, after "10" change "to" to and Signed and sealed this 21st day of March 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents (10-59) uscoMM-oc scan-ps9 fi LLS GOVERNMENT PRINTING OFFICE} l 0*IQIJI 

2. An aqueous solution according to claim 1 wherein said sugar acid is gluconic acid and said base is ammonia, and wherein said solution further comprises anions of a strong inorganic acid selected from the class consisting of hydrochloric and sulfuric acid in a concentration equivalent to between about 1 and 40 grams per liter of the corresponding ammonium salt.
 3. An aqueous solution according to claim 2 wherein said inorganic acid anions are chloride ions and wherein said solution further comprises from about 0.5 to 10 grams per liter of a compound selected from the group consisting of hydroquinone, o-cresotinic acid, beta-naphthol, ethylene gylcol, propylene glycol, water-soluble polyalkylene glycols and formamides.
 4. An aqueous solution according to claim 2 which further comprises from 2 to 10 grams per liter of dimethyl formamide.
 5. An aqueous solution according to claim 2 which further comprises from about 0.5 to 10 grams per liter of boric acid.
 6. An aqueous solution according to claim 1 wherein said sugar acid is a monocarboxylic polyhydroxy aliphatic acid of 5 to 7 carbon atoms and 4 to 6 hydroxyl groups, and is present in an amount between about 2.2 and about 3 moles per mole of tin in the solution; wherein said base is selected from the group consisting of ammonia and alkylolamines of 2 to 8 carbon atoms and is present in an amount sufficient to give a pH between about 6.5 and 7.5.
 7. A process for preparing a tin electrolyte solution which comprises dissolving in water (a) about 50 to 100 grams per liter of a tin salt selected from the group consisting of stannous chloride, stannous sulfate, stannous fluoborate and stannous fluoride, (b) 1.8 to 3 mole equivalents of a chelating acid selected from the group consisting of glyceric acid and C5 to C7 hydroxy carboxylic aliphatic acids containing 4 to 6 hydroxyl groups and 1 to 2 carboxyl groups per molecule per mole of stannous salt, and (c) sufficient base selected from the group consisting of ammonia and alkylolamines of 2 to 8 carbon atoms to adjust the pH of the solution to between about 6 and
 8. 8. A process according to claim 7 wherein said tin salt is stannous chloride, said sugar acid is gluconic acid and said base is ammonia.
 9. A process for preparing a tin electrolyte solution which comprises dissolving stannous chloride or sulfate in water, mixing the resulting stannous salt solution with ammonium hydroxide to precipitate stannous hydroxide from solution, mechanically separating the precipitated stannous hydroxide from the solution, redissolving the separated stannous hydroxide in an aqueous solution containing about 2.0 to 2.6 moles gluconic acid per mole of stannous hydroxide, and adding ammonium hydroxide to the resulting stannous gluconate solution in an amount sufficient to produce a substantially neutral solution characterized by a pH between about 6 and
 8. 10. A process according to claim 9 wherein the substantially neutral stannous gluconate solution is concentrated by evaporation.
 11. A process according to claim 9 wherein light colored stannous hydroxide is formed by adding the stannous salt solution to an agitated aqueous ammonia solution which contains sufficient ammonium hydroxide to convert the stannous salt to a precipitate of stannous hydroxide and give a pH between about 8.5 and 10 in the supernatant liquid.
 12. A process according to claim 9 wherein about 2 to 5 moles ethylene glycol is added to the stannous salt solution prior to mixing it with ammonia.
 13. A process for plating tin on an aluminum or aluminum alloy substrate which comprises immersing a tin anode and said substrate as a cathode in a plating solution which is essentially free from alkali metals and comprises stannous ions in a concentration of from about 10 to 70 grams tin per liter and, as an essentially sole chelating agent, a chelating sugar acid in a concentration equivalent of at least 1.8 mole equivalents of acid per mole of tin and a sufficient amount of ammonium hydroxide to give a pH between about 6 and
 8. 14. A process according to claim 13 wherein said sugar acid is a C5 to C7 monocarboxylic polyhydroxy aliphatic acid and electric current is set up between it and said anode to give a cathode current density of between about 10 and 100 amperes per square foot.
 15. A process according to claim 14 wherein said plating solution comprises stannous ions in a concentration of from about 30 to 60 grams tin per liter and ions of gluconic acid in a concentration equivalent to between about 2.2 and 4 mole equivalents of acid per mole of tin.
 16. A process according to claim 15 wherein an initial thin tin layer is deposited on said aluminum substrate from said plating solution by immersion and galvanic current action and further tin is deposited on said initial layer electrolytically in a system wherein said immersed aluminum substrate is connected as an anode and tin metal is connected as a cathode and current is passed through the system at a cathode current density of between about 10 and 30 amperes per square foot.
 17. A process according to claim 15 wherein the tin anode and the aluminum cathode have a ratio of surface areas of between about 2:1 and 3:1.
 18. A process according to claim 15 wherein the plating solution further comprises small effective amounts of ammonium chloride as a conductivity salt and a formamide as a brightener.
 19. A process according to claim 15 wherein the substrate is an aluminum wire and the current density is between about 40 and 100 amperes per square foot. 